The present invention relates to a process for producing a flat commutator and a commutator produced using this process. These commutators can be used especially in electric motors to drive a fuel pump which pumps fuels obtained from renewable raw materials.
In the production process disclosed in WO 97/03486, a metallic, pot-shaped carrier body forms segment support parts and is shaped from a copper plate. The copper plate has been segmented beforehand by grooves and is extruded with a hub formed from an electrically insulating molding compound. The carrier body, on its side forming the contact surface for the carbon-containing annular disk, is then removed to such an extent that the segment support parts are electrically separated from one another by the grooves filled with the molding compound. Then, the annular disk is applied and subsequently, according to the segmentation of the carrier body, divided into segments, the separating slots projecting into the area of the grooves which is filled with the molding compound.
Since in using the known process the carrier body is segmented before the annular disk is applied, the process requires additional steps to make grooves in the carrier body and remove the carrier body into the area of the grooves. Moreover, the dividing must take place precisely in the area of the grooves to ensure resistance to a reactive environment.
DE 36 25 959 C2 shows a drum commutator and a process for its production, in which either on a cylinder produced by curling a base plate of a parent or base metal, copper, or on a hollow cylindrical tube piece, protective parts are applied by plating with a copper-nickel or silver-nickel alloy, at least on the surfaces which come into contact with the brushes. Furthermore, the parent metal of the commutator segments is provided on its surface with tin plating by electrolytic plating (column 13, lines 16 and 17) to prevent the copper body from being exposed to a fuel, such as gasohol, to prevent decomposition of the fuel. A mixture of unleaded gasoline and 10 to 15% ethyl alcohol is defined as gasohol in the patent.
DE 44 35 884 C2 shows a commutator for use in fuel pumps, with bars located around the periphery of the commutator and in sliding contact with a brush arrangement, of a wear-resistant copper-magnesium alloy. The magnesium portion of the bars is between 0.05 and 2.00 percent by mass.
In contrast to this invention, JP 58 075440 A does not disclose a flat commutator, but a drum commutator. Furthermore, this document is directed at the prevention of fuel oxidation (xe2x80x9cto prevent the oxidation of gasolinexe2x80x9d). To this end, a plate (sheet 8) resistant to fuel is connected with the not yet burnished copper plate forming the carrier body.
FR2 330 169 A also discloses a drum commutator (cf. FIGS. 1 to 3) and hence a nongeneric subject. The layer with reference numbers 11a and 11b depicted in FIG. 5 of this document is a layer produced by oxidation.
U.S. Pat. No. 5,175,463 discloses a flat commutator with segment support parts separated by radial slots. A compound with low melting point of different metals is used in the connection of the carbon-containing annular disk with the metallic segment support parts.
DE 29 03 029 C2 represents the proximate state of the art and discloses among others a process for producing a flat commutator in which a copper plate with a disk-shaped sheet of silver or silver alloy invulnerable to gasoline is applied. The copper plate is sloted at regular intervals. The denuded copper parts of the commutator bars are covered with a galvanically applied electroplated layer of silver or tin.
Objects of the present invention are to provide a process for producing a flat commutator which eliminates the disadvantages of the prior art, which in particular is more economical, and which still ensures sufficient resistance of the finished commutator in a reactive environment. In addition, the coating will be relatively thick, especially in undercuts and/or grooves which may be present as a result of dividing the carrier body, and will be as uniform as possible. In any case, it will be possible to apply the coating to form a cohesive layer. The present invention permits use of electric motors for driving a pump for fuels obtained from renewable raw materials.
The surfaces of the metallic segment support parts, which are exposed by dividing, are covered with a coating which is resistant to a reactive or aggressive environment. The resistance relates especially to protection of the carrier body and/or the segment support parts and the connection to the annular disk against breakdown, relates to electrical conductivity with respect to the contact resistance between the commutator contact surface formed by the annular disk and the pertinent segment support part or between it and the commutator brush, and relates to the adhesion of the coating on the metallic segment support part. Also, insulation must be ensured between the segment support parts. The segment support parts preferably and essentially consist of copper and have high electrical conductivity and ductility. The carrier body is produced, for example, from a punched-out copper plate which is then formed into a pot and is extruded with a molding mass forming the hub. The carbon-containing annular disk in particular is resistant in a reactive environment, for example in a hydrocarbon-containing liquid. The annular disk and/or the carrier body is/are divided preferably by abrasive cutting, sawing or laser working.
The process steps of forming the grooves and removing the carrier body are eliminated by the carrier body being divided into segment support parts after joining to the annular disk.
Production is further simplified by the annular disk and the carrier body being divided in one step. Alternatively, it is possible in a first step to divide the carrier body, provided with the hub and formed into a pot, into segment support parts by first slots. Then, the annular disk is applied. Finally, the annular disk is divided by two slots into annular segments, the second slots preferably being smaller than the first slots and being located within the first slots. The coating of the surfaces of the segment support parts exposed by dividing the carrier body can be done before or after the application of the annular disk. To the extent the coating takes place before applying the annular disk, the applied layer can be used at the same time as a joining layer to the annular disk.
Because coating takes place by deposition, the metallic carrier body can be coated with any material. Both chemical and also physical and mixed deposition processes can be used, for example deposition from the gaseous phase (Chemical Vapor Deposition, CVD), optionally plasma- or laser-supported, cathode beam atomization (sputtering), vapor deposition, etc. Vossen, Kern (publisher): Thin Film Processes I and II, 1991, surveys possible deposition methods.
Because deposition takes place from a solution or suspension, a large number of commutator elements can be coated in one step, and thus, economically and with good coverage and layer quality. The layer material is in a preferably an ionic solution or suspension and can be deposited electrolytically (galvanically) or currentlessly on the segment support parts.
Because deposition takes place currentlessly from the solution or suspension, i.e. without applying an external voltage, coverage of the elements even on inaccessible locations, for example in the dividing slots formed by division, is good. The temperature and concentration of the solution or suspension are chosen such that complete coverage with sufficient thickness is ensured in as short a time as possible.
Because coating takes place selectively only on surfaces of the segment support parts, the annular disk and especially the hub are not coated, preventing the detachment of the layer from these locations, for example due to poor adhesion, and the associated problems in later operation of the commutator. The selectivity of deposition can be adjusted by the corresponding choice of the process parameters during deposition, for example the deposition temperature, concentration of the solution or suspension, deposition duration, etc., depending on the material to be deposited and/or the carrier body to be coated.
Because coating takes place with tin, silver or chromium, good coverage and adhesion as well as sufficient resistance especially to fuels obtained from renewable raw materials is also ensured with economical materials. Tin in particular offers good contact properties, and is also advantageous for joining the winding ends to the segment support parts.
Because the layer thickness is between 0.1 and 10 xcexcm, especially between 1 and 3 xcexcm, reliable coating and good adhesion as well as sufficient resistance are guaranteed. These layer thicknesses arise especially in currentless deposition from a solution or suspension after comparatively short deposition intervals and ensure pore-free coverage of the carrier body.
In a commutator produced using the process of the present invention, the hub in the area of the division, especially on the side of the segment support parts facing away from the commutator contact surface and/or the surfaces adjoining the surfaces exposed by the division of the carrier body, also adjoins the carrier body. Thus, reliable coverage of the metallic carrier body is also ensured in this area. This coverage prevents scouring of the carrier body and the segment support parts in a reactive atmosphere.
Because the hub forms a complete cover of the cylindrical boundary surface of the central hole of the carrier body, the cylindrical inside of the carrier body is also covered relative to the reactive atmosphere. Also, the resistance of the commutator is further increased.
Because the coating is resistant to the fuel to be pumped, commutators produced using the process of the present invention can also be used in fuel pumps. Especially, tin as the coating material has proven resistant to fuels obtained from renewable raw materials, for example alcohol-based fuels or diesel fuels obtained from rapeseed oil.
Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the annexed drawings, discloses preferred embodiments of the present invention.